This invention relates to a method of and apparatus for producing a product cryogenic liquid mixture comprising oxygen and nitrogen having a chosen mole fraction of oxygen.
EP-A-0 657 107 discloses that a combined mixture of liquid oxygen and a liquid nitrogen having a chosen mole fraction of oxygen less than the mole fraction of oxygen in natural air is particularly useful in providing, on evaporation, a breathable refrigerating atmosphere. Producing such a liquid cryogen therefore requires the separation of oxygen and nitrogen from air, typically in one or more cryogenic rectification columns, followed by the remixing of the two gases. A considerable amount of work needs to be expended in order to separate the air. Only a relatively small proportion of this work can be recovered when the two gases are remixed.
The present invention relates to an improved method and apparatus for producing a product cryogenic liquid mixture comprising oxygen and nitrogen having a chosen mole fraction of oxygen.